Field of the Invention
The present invention relates to a display device, and more particularly, to an image processing method and circuit, and a display device using the same, for minimizing image quality degradation of a high dynamic range (HDR) image and displaying the image in a standard dynamic range (SDR) display device.
Discussion of the Related Art
In general, a captured image is required to be digitized in order to display the image on a display device. In this case, gamma encoding and gamma decoding processes are required. The gamma encoding is used to contain a large amount of information in a given bandwidth (e.g., a 8-bit image signal has a gray scale of 256), is relatively sensitive to change in brightness in a low luminance period compared with a high luminance period according to human vision cognitive characteristics, that is, having nonlinear characteristics. In consideration of this, a nonlinear transfer function is used during gamma encoding and is defined according to Recommendation (Rec.) 709 and Rec. 1886 standard that use a reciprocal of 2.4 as an index. The display device determines a gamma reference voltage in consideration of a function having, as an index, 2.4 as a reverse function of a transfer function used in encoding in order to convert a gamma encoded image into originally intended luminance for each gray scale.
A display device that is considered according to the conventional Rec. 709 standard is a cathode-ray tube (CRT), and thus the device has a narrow dynamic range of about 0 to 100 cd/m2. However, 2.4 is proper to a dynamic range of the CRT, and thus, when the dynamic range is increased, 2.4 is not appropriate to human vision cognitive characteristics. In reality, the human has a wide dynamic range of about 10−4-108 cd/m2. Technology in consideration of this is a high dynamic range (HDR) and thus far, the HDR technology has been mostly concentrated on camera fields.
Recently, there has been movement for expanding HDR to film production, display development, etc., and Society of Motion Picture and Television Engineers (SMPTE) standard (ST.) 2084 standard, Blu-ray Disc Association (BDA) HDR standard, etc. have been representatively established and discussed. The SMPTE ST. 2084 standard refers to an electro-optical transfer function (EOTF) for encoding an HDR image for an HDR display device and is also referred to as a perceptual quantizer (PQ).
Ad described above, the gamma encoding is used to contain a large amount of information in a given bandwidth if possible and decoding is a process for converting encoded information into an original brightness expression. Accordingly, encoding and decoding have a relationship of a reverse function, and thus when encoding and decoding functions are different, image quality degradation is caused.
That is, although an HDR image needs to have higher image quality than a standard dynamic range (SDR) image, when the HDR image is displayed in a conventional SDR display device, image quality of the HDR image is degraded compared with the SDR image due to different decoding and decoding functions.
This is because most conventional SDR display devices decode an images using gamma defined according to the conventional SDR standard (Rec. 709/Rec. 1886) and thus do not decode the HDR image encoded according to the HDR standard (ST. 2084), which is not overcome even if a dynamic range of the display device is increased.
On the other hand, in the case of a display device that is conformable to the HDR standard (ST. 2084), an HDR image is accurately displayed but an SDR image is not accurately displayed.
In order to overcome these problems, a decoding function that accurately corresponds to a transfer function of encoding an image needs to be embodied in a display device. Accordingly, in order to appropriately display both an SDR image and an HDR image in terms of a display device, it is most ideal that respective decoding functions EOTF of SDR and HDR are embodied in a data drive IC, but there is a problem in terms of high cost.